Main Belt Asteroid Science in the Decade 2023-2032: Fundamental Science Questions and Recommendations on behalf of the Small Bodies Assessment Group Authors: Maggie M. McAdam (1), Andrew S. Rivkin (2), Lucy F. Lim (3), Julie Castillo-Rogez (4), Franck Marchis (5), Tracy M. Becker (6) Cosigners: Edgard G. Rivera-Valentín (7), Maitrayee Bose (8), Michelle Thompson (9), Conel M. O'D. Alexander (10), Jamie L. Molaro (11) Michael C. Nolan (12) Noam R. Izenberg (2), Henry H. Hsieh (11), Caitlin Ahrens (13), Jessica Noviello (8), Cristina Thomas (14), Hannah Kaplan (3), Michael S. Bramble (4), Meenakshi Wadhwa (8). Affiliations: 1. NASA Ames Research Center 2. Johns Hopkins University Applied Physics Laboratory 3. NASA Goddard Space Flight Center 4. Jet Propulsion Laboratory, California Institute of Technology 5. SETI Institute 6. Southwest Research Institute 7. Lunar and Planetary Institute (USRA) 8. Arizona State University 9. Purdue University 10. EPL, Carnegie Institution of Washington 11. Planetary Science Institute 12. Lunar and Planetary Laboratory, University of Arizona 13. University of Arkansas 14. Northern Arizona University Lead Authors Contact Information: Maggie M. McAdam Building N232 NASA Ames Research Center, Moffett Field,CA 94035 Ph: 603-305-3783 [email protected] Andrew. S Rivkin JHU Applied Physics Laboratory Ph: 240-228-2811 [email protected] 0 Executive Summary: Solicited by the Small Bodies Assessment Group, we present and discuss the small body science community’s fundamental questions and recommendations for Main Belt asteroid science in the upcoming decade. We briefly and non-exhaustively review some of the breakthroughs in our field during the last decade and discuss what important science still remains. The fundamental questions facing Main Belt asteroid science fall into three groups: physical properties and processes, chemical composition and evolution and dynamical evolution. We recommend a balanced program of telescopic observation (ground-based, airborne, and space- based), laboratory studies, theoretical research and missions to Main Belt Asteroids utilizing the full spectral range from ultraviolet to far-infrared to investigate these outstanding fundamental questions in the next decade. Background and Motivation The decade of the 2010s saw great advancements in our understanding of solar system formation and evolution. Observations from telescopes and spacecraft combined with laboratory geochemical measurements of meteorites and collisional and dynamical modeling Figure 1: Main Belt asteroids synthetic proper elements (first 50,000 numbered of individual objects bodies; Novakovic, Knezevic and Milani June 2017). Here we have plotted the known asteroid in the Main Belt by semi-major axis and sine(inclination). The size and populations give of the point indicates the size of the object. us greater insight into the overall story of how our planetary system was put together, a story which features main-belt asteroids as key witnesses and objects for further study. Our emerging understanding is that planetesimals formed directly from cm-scale pieces into objects of 50-200 km diameter. Meteoritical evidence shows that these planetesimals seem to have formed in at least two distinct reservoirs, with the formation of Jupiter likely to have separated two of these reservoirs [1, 2]. Some of these planetesimals continued to grow, forming planetary embryos (or “protoplanets”), some of which experienced additional accretion to form the terrestrial planets. Planetary migration served to scatter and transport unaccreted planetesimals and planetary embryos, leaving roughly 150-200 in stable orbits between Mars and Jupiter [3]. While subsequent collisions and orbital evolution via gravitational and non-gravitational forces resulted in the millions of objects in the main asteroid belt, these objects are thought to derive from this relatively 1 small number of original planetesimals, some of which remain intact and available for exploration today. The last decade saw several important advancements in our understanding of Main Belt asteroids and their relevance to the Solar System’s history and evolution. We will briefly describe some of these important insights. This is by no means an exhaustive list and are not listed in any particular order. Missions: Ceres and Vesta. The Dawn mission investigated protoplanet Vesta (2011-2012) and dwarf planet Ceres (2015-2018) via orbital encounters that produced near global mapping of these bodies with visible imaging, infrared spectroscopy, elemental spectroscopy, and gravity science [4]. Dawn was the first spacecraft to use solar electric propulsion for a planetary science mission. Key results at Vesta include a better understanding of early planetary differentiation as a consequence of short-lived radioisotope decay. At Ceres, Dawn discovered a relict ocean world and potentially ongoing brine effusion [5, 6]. Spectroscopic and elemental measurements at Vesta confirmed earlier conclusions that Vesta is consistent with the properties expected of the HED parent body [e.g., 7] although the definitive identification of Vesta as “the” HED parent body, or indeed any single asteroid to a group of meteorites, may be an oversimplification. These same techniques at Ceres demonstrated the existence of regolith with latitude-dependent ice content and an interpretation of a CI-like surface composition [8], with the complication of a widespread ammoniated component that has not been found in any meteorite samples [8, 5]. Spectral measurements by Dawn have also detected high local concentrations of organic materials on Ceres [9, 10, 11]. The Asteroidal-Cometary Continuum: The neat division of small bodies into the rocky asteroids and the icy comets began to be blurred in the later years of the previous decade, and in this decade evidence accumulated that a large number of objects in the main belt are at least somewhat icy in nature. We discussed Ceres above, and observations of 10 Hygiea by multiple groups [12, 13] have found it to have a spectrum in the 3-µm region very similar to that of Ceres, implying similar hydrated minerals and volatiles and perhaps a similar history and nature. Absorption bands on 24 Themis [14, 15] and 65 Cybele [16] were interpreted as due to water ice frost and organic materials, and several additional objects with similar band shapes have been found in the last decade [17, 12]. Spectra of 324 Bamberga showed greater similarity to comet 67P, the target of the Rosetta Mission, than any other asteroids in the 3-µm region [13]. The spectral evidence for icy asteroids was reinforced by observations of coma on some small asteroids. At least some of these objects (but not all) appeared to have activity driven by sublimation, implying near-surface volatiles and recent exposure. Grand Tack Dynamical Model: Dynamical studies over the past decade have shown that small bodies both are tracers of planetary migration early in solar system history and were drivers of it [e.g., 18]. In the “Grand Tack” scenario, inner-solar-system planetesimals and outer-solar- system planetesimals are transported both into and out of the present-day asteroid belt, with an implication that low-albedo, volatile-rich objects were originally formed among the giant planets 2 (and the relatively rare D-class asteroids were transported from the transneptunian region) while the higher-albedo, volatile-poor objects were originally formed sunward of Jupiter. The NEOWISE survey demonstrated that low-albedo material dominates the asteroid belt [19, 20], though the spectral evidence discussed in the previous section shows that low-albedo objects have diverse compositions. Primordial Families: Identification and interpretation of asteroid dynamical families were subject to great advances during the last decade, thanks in part to both an increase in observational data [e.g., 21] as well as a greater appreciation of the Yarkovsky Force [22, 23]. Recently, multiple authors have proposed that large subsets of the main belt population derive from a very small number of parent bodies: Delbo et al. [3] concluded that practically all low-albedo objects in the inner Main Belt belong to a single “Primordial Family”, with a similar family uniting most medium-albedo, X-complex objects in the middle belt [24]. Dermott et al. [24] found that 85% of inner-belt objects can be traced to one of only five families. These findings suggest that a comprehensive understanding of the compositions of the vast majority of main-belt asteroids may be undertaken via investigations of a relatively small number of objects, which based on other work quoted above likely represent original planetesimals. Non-Sublimating Active Asteroids and Their Drivers: The greater understanding of the Yarkovsky Force comes hand-in-hand with greater understanding of other non- Figure 2: Asteroid (596) Scheila with impact driven activity. Image credit: gravitational forces and NASA, ESA and Jewett. their influence on asteroid geophysics. The YORP torque has been identified as the likely cause for asteroid satellites and possibly the “top-shapes” that appear to be common in ~km-scale near-Earth objects [e.g., 26]. YORP spin-up and disruption/fission events also appear to be the cause of activity in most of those active asteroids that are not experiencing sublimation. The number of known MBAs is large enough, and their monitoring (via all-sky surveys) frequent enough, that discovery and observation of these events have become relatively routine.